docs: document trace_config parameter in Span.__init__

The trace_config parameter was unused.
Originally intended for trace-level configuration from the OpenTelemetry protocol,
but the upstream TraceConfig proto was removed. Retained for backwards compatibility.

Author: samyak1729

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -786,7 +786,9 @@ class Span(trace_api.Span, ReadableSpan):
         parent: This span's parent's `opentelemetry.trace.SpanContext`, or
             None if this is a root span
         sampler: The sampler used to create this span
-        trace_config: TODO
+        trace_config:  Unused. Originally intended for trace-level configuration
+            from the OpenTelemetry protocol, but the upstream ``TraceConfig``
+            proto was removed. Retained for backwards compatibility.
         resource: Entity producing telemetry
         attributes: The span's attributes to be exported
         events: Timestamped events to be exported